The long-term objectives of this research are to define the genetic and cellular events that characterize colonization of animal epithelial tissue by beneficial bacteria. The specific symbiotic infection of the light-emitting organ of the squid Euprymna scolopes by the luminous bacterium Vibrio fischeri will be used as a model system through which to address these objectives. Recent investigations have centered on the events characterizing the initiation and persistence of the association using newly hatched, uncolonized juveniles, and symbiotic adult squid. Experiments use bacterial mutants and host cellular events to manipulate and assay the complex succession of signaling and responses by which the host and bacterium recognize each other. Recently developed genomic tools, including bacterial and host microarrays based on the V. fischeri genome sequence and a 13,000 member squid EST gene library, provide an exceptional opportunity to understand the genetic mechanisms underlying the initiation and persistence of this beneficial association. The proposed research plan integrates 3 specific aims: 1. To examine the role of reciprocal signaling activities as the bacterium first encounters the host 2. To understand the interactions between the symbionts and host phagocytes and epithelial cells 3. To determine the mechanisms driving the daily expulsion and regrowth of the symbionts. This system serves as a model of both beneficial and pathogenic Vibrio infection, as well as the evolution of virulence in vibrios. Specifically, genomic comparisons have already revealed shared colonization/virulence factors important to both symbionts and pathogens. The work proposed here will also provide new information about how specific, stable associations with aquatic invertebrates may serve as environmental reservoirs for human pathogenic Vibrio species.